1. Field of the Invention
The present invention relates to a plastic optical fiber with a lens portion having a function of controlling light rays, such as a light-condensing function, in which a concave recess such as a spherical recess is formed on an end face of the plastic optical fiber, and a refractive index adjusting material or filler (including air or the like) with a refractive index different from (typically larger than) that of the plastic optical fiber being filled in the concave recess. The present invention also relates to an optical fiber connector using the plastic optical fiber, a connecting structure or method using the plastic optical fiber, a light-emitting/receiving apparatus (this term means a light-emitting or light-receiving apparatus in this specification) in which a light-emitting/receiving device (this term means a light-emitting or light-receiving device in this specification) is combined with the plastic optical fiber with a lens portion, and the like.
2. Related Background Art
In recent years, in order to enhance the coupling efficiency between plastic optical fibers, or a plastic optical fiber and a light-emitting device/receiving device, there have been proposed some methods of forming a convex lens at the end face of the plastic optical fiber. For example, Japanese Patent Application Laid-Open No. 10(1998)-239538 discloses a method of forming a spherical contour on the end face of a plastic optical fiber by using a solvent, Japanese Patent Application Laid-Open No. 11(1999)326689 discloses a method of forming a spherical contour on the end face of a plastic optical fiber by immersing the end thereof in an organic solvent containing an optical fiber material and drying the end face after the optical fiber is lifted from the solvent, Japanese Patent Application Laid-Open No. 5(1993)-107427 discloses a method of forming a spherical contour on the end face of an optical fiber by immersing the end thereof in a photosensitive resin and hardening the end face after the optical fiber is lifted from the resin, Japanese Patent Application Laid-Open No. 8(1996)-75935 discloses a method of forming a lens shape on the end face of an optical fiber by pressing the end face thereof against a heated lens-forming mold, and Japanese Patent Publication No. 62(1988)-57001 discloses a method of forming a spherical surface on the end face of an optical fiber by heating and softening the end thereof, using its surface tension.
Further, there have been proposed, for fabricating a concave contour on the end face of a plastic optical fiber with a refractive index distribution, a method of molding the end face of an optical fiber by a heated metal mold, and a method of solving the end face of an optical fiber by a solvent (see Japanese Patent Application Laid-Open No. 11(1999)-242129).
However, end faces of those optical fibers all have convex contours, and hence, alignment between such an optical fiber and a light-emitting/receiving device is hard to achieve, compared to the case of an optical fiber with a flat end face. Further, when the end face of the optical fiber is caused to abut on an optical device, a high pressure is likely to be applied to a portion of the device, and therefore, there is a considerable possibility of damaging the device.
Moreover, the transmission efficiency in long-distance optical transmissions using optical fibers is greatly influenced by coupling losses at connecting portions between optical fibers and between an optical fiber and a light-emitting/receiving device. The coupling loss is due to deviation of the optical axis, light scattering on the end face of the optical fiber, and the like. A variety of connecting methods using light-condensing lenses have been conventionally proposed to reduce such coupling loss.
On the other hand, a large-diameter plastic optical fiber that can be readily fabricated at a relatively low cost has been recently developed, and is used in medium and short distance networks. Where those local networks are connected to a trunk-line network, there is a need of performing connection between transmission light from devices with different core diameters or numerical apertures, such as between large-diameter optical fiber and crystal or silica-contained optical fiber. The numerical aperture is determined by refractive indices of the medium around the optical fiber, its core and its cladding, while the core diameter is a physical size of the core of the optical fiber. Therefore, those terms have different categories. Problem occurs in the connection between optical fibers of which at least one of the numerical aperture and the core diameter is different.
As a method of obtaining a high coupling efficiency in such a connecting portion, there have been conventionally proposed structures as illustrated in FIGS. 1 and 2. In FIG. 1, there is illustrated a numerical-aperture converting structure using a light-condensing lens 403 such as a ball lens and a rod lens (see Japanese Patent Application Laid-Open Nos. 60(1985)-61707 and 5(1993)-34545). In FIG. 2, there is shown a numerical-aperture converting structure using a lensed optical fiber 414 whose end face is shaped into a spherical convex contour. Representative optical rays are indicated in FIGS. 1 and 2. Conversely, there has also been proposed a method in which a concave lens or the like is attached to the end face of an optical fiber with a smaller numerical aperture.
In the structure of FIG. 1 having three elements or more, however, there is the problem of deviation and inclination between optical axes of optical fibers 402 and lens 403. Particularly, alignment of a distance between end faces of the optical fibers 402, and fixation of the lens 403 are difficult, and resistance of fixture thereof to external shocks is small. In FIG. 2, the possibility of deviation in the optical system is lowered since a light-condensing lens is integrated with the lensed optical fiber 414 and the number of optical devices (402 and 414) is hence reduced to two. However, it is difficult to cause the end face of the optical fiber 414 to abut on a spacer and fix the optical fiber 414 since its end face is spherical.
It is an object of the present invention to provide a plastic optical fiber with a lens portion having a function of controlling light rays, such as a light-condensing function, in which a concave recess such as a spherical recess is formed on an end face of the plastic optical fiber, and a refractive index adjusting material or filler with a refractive index different from (and typically larger than) that of the plastic optical fiber is filled in the concave recess, an optical fiber connector using the plastic optical fiber, a connecting structure or method using the plastic optical fiber, a light-emitting/receiving apparatus in which a light-emitting/receiving device is combined with the plastic optical fiber with a lens portion, and the like.
In this specification, the plastic optical fiber means an optical fiber whose core and cladding are made of a polymer, or an optical fiber whose core is made of a polymer. In such a plastic optical fiber, the cladding may be covered with a protective layer, or a polymer jacket. Further, the plastic optical fiber may be a step-index (SI) type wherein a refractive index along its radial direction is uniform throughout but exhibits an abrupt step at its core-cladding interface, a graded-index (GI) type wherein a refractive index varies in some continuous fashion as a function of radial distance, or the like.
According to one aspect of the present invention, there is provided a plastic optical fiber with a lens portion which includes a plastic optical fiber with a concave portion formed on its end face, and a lens portion having a function of controlling light rays. The lens portion is formed of a refractive index adjusting material filled in the concave portion and having a refractive index different from a refractive index of the plastic optical fiber.
On the basis of the above structure, the following more specific structures are possible.
The concave portion can have a configuration of a rotation-symmetrical shape, such as a spherical shape, about an optical axis of the plastic optical fiber. The concave portion may also have an aspherical shape, when necessary.
The refractive index adjusting material typically has a refractive index larger than the refractive index of the plastic optical fiber to form the lens portion as a light-condensing lens portion.
The refractive index adjusting material is curable or non-curable, and an outer surface of the refractive index adjusting material filled in the concave portion can be substantially flat or spherical. The light controlling function can be obtained not only at the adjusting material-fiber interface but at the adjusting material-air interface. Therefore, large light-condensing power or the like can be achieved even by a plastic optical fiber with a flat end face or an end face of a large radius of curvature, and hence, connection and alignment between the plastic optical fiber and a light-emitting/receiving device or another optical fiber can be performed more readily than a plastic optical fiber with a considerably-convex end face.
A flat portion can be formed around the concave portion on the end face of the plastic optical fiber.
According to another aspect of the present invention, there is provided a plastic optical fiber with a lens portion which includes a plastic optical fiber, a concave portion formed on an end face of the plastic optical fiber and having a function of controlling light rays, and a flat portion formed around the concave portion on the end face of the plastic optical fiber. When this plastic optical fiber is used, a filler, such as non-curable refractive index material, air, or nitrogen gas, is filled in the concave portion and its surroundings.
According to yet another aspect of the present invention, there is provided a plastic optical fiber with a lens portion which includes a plastic optical fiber with a concave portion formed on its end face, and a lens portion having a function of controlling light rays. The lens portion is formed of a refractive index adjusting material filled in the concave portion and having a refractive index larger than a refractive index of the plastic optical fiber. Generally the refractive index of the plastic optical fiber is small (about 1.3), so its light-condensing efficiency is not sufficient when it is optically connected to another device. A sufficient light-condensing efficiency can be obtained by the lens portion of a large refractive-index adjusting material filled in the concave portion.
According to yet another aspect of the present invention, there is provided a plastic optical fiber with a lens portion which includes a plastic optical fiber with a concave portion formed on its end face, and an optical fiber which is different from the plastic optical fiber in at least one of a numerical aperture and a core diameter. The plastic optical fiber and the optical fiber are arranged with their end faces opposed to each other, a spacing interposed between their end faces, and optically coupled to each other.
Typically, the plastic optical fiber is larger than the optical fiber in at least one of the numerical aperture and the core diameter.
The concave portion of the plastic optical fiber can be filled with a curable or non-curable filler having a refractive index larger than a refractive index of the optical fiber.
The concave portion of the plastic optical fiber and its surroundings can also be filled with a curable or non-curable filler having a refractive index larger than a refractive index of the optical fiber.
When the concave portion and its surroundings are filled with air or gas whose refractive index is smaller than a refractive index of the core of the plastic optical fiber, the plastic optical fiber and the optical fiber are optically coupled in a direction from the optical fiber to the plastic optical fiber.
When a flat portion is formed around the concave portion, the flat portion can be caused to abut on a spacer such that the plastic optical fiber is aligned in an optical-axial direction.
According to yet another aspect of the present invention, there is provided an optical fiber connector which includes a plastic optical fiber with a concave portion formed on its end face, an optical fiber that is different from the plastic optical fiber in at least one of a numerical aperture and a core diameter, a first ferrule for holding the plastic optical fiber, a second ferrule for holding the optical fiber, and a housing member for housing the first ferrule and the second ferrule to arrange the plastic optical fiber and the optical fiber with their inner end faces opposed to each other and a spacing formed between their inner end faces, and to couple optically the plastic optical fiber and the optical fiber.
The housing member can be a pair of semi-cylindrical split sleeves, or an integral cylindrical member.
Outer end faces of the plastic optical fiber and the optical fiber can be formed to lie in the housing member, or on a plane which is the same as an end face of the housing member. Further, the outer end faces of the plastic optical fiber and the optical fiber can be formed non-parallel or parallel with the end face of the housing member, or uneven.
The optical fiber connector can further include a connecting unit, such as a threaded cylindrical member, for optically connecting an optical device to the optical fiber.
The outer end faces of the plastic optical fiber and the optical fiber can be formed to lie outside the end face of the housing member.
A filler, such as a curable or non-curable resin, can be filled in the spacing.
Further, a spacer can be contained in the housing member and inserted between inner end faces of the plastic optical fiber and the optical fiber.
According to yet another aspect of the present invention, there is provided a connecting method of connecting optical fibers in which a plastic optical fiber with a concave portion formed on its end face, and a second optical fiber are prepared, the concave portion of the plastic optical fiber is filled with a refractive index adjusting material, such as a resin adhesive, end faces of the plastic optical fiber and the second optical fiber are opposed to each other, and the end faces of the plastic optical fiber and the second optical fiber are bonded to optically couple the plastic optical fiber and the second optical fiber to each other.
The second optical fiber can also be a plastic optical fiber with a concave portion formed on its end face, which is filled with the refractive index adjusting material. The second optical fiber can be a plastic optical fiber with a flat end face, or a silica-contained or crystal optical fiber.
According to yet another aspect of the present invention, there is provided a connecting structure between optical fibers which includes a plastic optical fiber with a concave portion formed on its end face, a second optical fiber, and a refractive index adjusting material, such as a resin adhesive, filled in the concave portion of the plastic optical fiber. End faces of the plastic optical fiber and the second optical fiber are opposed and bonded to each other to optically couple the plastic optical fiber and the second optical fiber to each other.
The refractive index adjusting material is typically hardened finally. When the refractive index adjusting material is non-curable, an adhesive needs to be applied to a periphery of the opposed optical fibers to bond these optical fibers.
According to yet another aspect of the present invention, there is provided a connecting structure of a plastic optical fiber and a light-emitting/receiving device which includes a plastic optical fiber with a lens portion having a function of controlling light rays, a light-emitting/receiving device, and a substrate with a holding structure for holding the plastic optical fiber with the lens portion formed at its end face and an arranging portion for arranging the light-emitting/receiving device thereon. The lens portion is formed of a refractive index adjusting material filled in a concave portion on an end face of the plastic optical fiber and having a refractive index different from a refractive index of the plastic optical fiber. The plastic optical fiber is held in the holding structure to align the lens portion to the light-emitting/receiving device arranged on the arranging portion and optically couple the plastic optical fiber and the light-emitting/receiving device.
The holding structure can be a recess with a diameter slightly larger than a diameter of the plastic optical fiber, the arranging portion can be a bottom face of the recess, and a distance between the plastic optical fiber and the light-emitting/receiving device can be adjusted by controlling an insertion depth of the plastic optical fiber into the recess.
The holding structure can also be a stepped recess with an upper opening portion with a diameter slightly larger than a diameter of the plastic optical fiber and a lower opening portion with a diameter smaller than the diameter of the plastic optical fiber and a predetermined depth. The arranging portion can be a bottom face of the stepped recess, and a distance between the plastic optical fiber and the light-emitting/receiving device can be adjusted by causing the end face of the plastic optical fiber to abut a stepped face formed between the upper opening portion and the lower opening portion.
The refractive index adjusting material can be non-curable, and the non-curable refractive index adjusting material can be filled in the surroundings of the light-emitting/receiving device.
According to yet another aspect of the present invention, there is provided a method of optically connecting a plastic optical fiber to a light-emitting/receiving device in which a substrate is prepared with a holding structure for holding a plastic optical fiber with a lens portion formed at its end face and an arranging portion for arranging a light-emitting/receiving device thereon, the light-emitting/receiving device is positioned on the arranging portion, and the plastic optical fiber is set in the holding structure to oppose the lens portion to the light-emitting/receiving device and optically couple the plastic optical fiber and the light-emitting/receiving device.
These and other advantages will be more readily understood in connection with the following detailed description of the more preferred embodiments in conjunction with the drawings.